3D model: Koshikake-ari-tsugi by thejoinery on Sketchfab
Assembly insights
Used in modular architecture, this joint offers a practical balance between strength and efficient fabrication, locking components together without visible hardware. The lap dovetail combines the speed of a half-lap with the mechanical lock of a dovetail, making it ideal for lengthening beams and joining modular pavilion components that must be assembled on site.
Historical context
Koshikake Ari Tsugi sits between simple scarfs and full temple-grade tsugite. The "koshikake" (sitting lap) profile provides a broad bearing surface while the dovetail resists pull-out. Carpenters use it when they need more security than Kiritsuke Tsugi but less complexity than Kanawa Tsugi. It appears in regional house framing and contemporary timber structures inspired by traditional methods.
Construction steps
- Mark the lap depth (typically half the timber thickness) and dovetail angle on both pieces.
- Saw the lap face on each member, then lay out the dovetail profile on the end grain.
- Cut the dovetail pin and tail on the lapped sections using a dozuki or ryoba saw.
- Chisel the waste; test-fit until the lap seats flush and the dovetail locks.
- Optional: add a draw pin or key for extra security on structural applications.
Where to see this joint in Japan
- Regional farmhouses and barns — Rural Japan — beam extensions and repairs
- Contemporary timber pavilions — Japan and abroad — modular joinery projects
Variations
Lap depth and dovetail angle can be adjusted for timber size and load. Some versions use a single tail; others use multiple tails on wide beams. Related joints include Kone Tsugi (shouldered scarf) and Ari Tsugi (full dovetail splice).
Applications
- Long-span beams
- Modular pavilions
- Furniture rails
Tools required
- Ryoba saw
- Chisels
- Wood mallet